The goaf water accumulation area has become the primary hazard source of hidden disaster in coal mine, and it is an important factor of hidden disaster in coal mine safety production. The ground transient electromagnetic method (TEM) is sensitive to low-resistance water bodies, with high resolution and flexible construction, which is the mainstream technology for the detection of goaf water accumulation areas. However, complex terrain can cause electromagnetic field distortion, abnormal distortion of apparent resistivity, and abnormal position offset, which seriously reduces the interpretation accuracy. Coal mines in mountainous areas of central and western China generally have complex exploration conditions including severe terrain undulation, vertical and horizontal valleys, and strong human electromagnetic interference. Taking the detection of goaf water accumulation area as an engineering example, this paper systematically expounded the electrical difference basis, method principle, field construction optimization, whole process data processing and terrain correction key technologies of transient electromagnetic method under complex mountainous terrain, focusing on solving the false abnormal problem of ' every ditch must be low and every mountain must be high ' caused by terrain undulation. Through the use of single-point coefficient method terrain correction, three-dimensional elevation constraint inversion, anti-interference parameter optimization, low-resistance abnormal threshold statistics and other technical means, the terrain distortion and electromagnetic interference were effectively eliminated. Combined with the comprehensive interpretation of geological and hydrological data, the detection reliability of goaf water accumulation area was effectively improved, which provided a reliable geophysical basis for the survey of hidden disaster-causing factors, advanced control of water disaster and safe mining in goaf water accumulation area of coal mine.